The invention relates to a computer apparatus such as a lap-top type personal computer or the like having a disk device as an external storage device and, more particularly, to a computer apparatus which can be also used by an internal power source by a battery provided in the apparatus in addition to an external commercially available AC power source and, further, which has a power saving function to reduce an electric power consumption that can be realized by an OS of a computer and also relates to an external storage device.